1. Field of the Invention
This invention relates to the field of ultra wideband communications and radar. More particularly, it relates to reliable reception and processing of ultra wideband electromagnetic pulses in the presence of noise, strong interference and/or jamming.
2. Background of Related Art
One of the first ultra wideband (previously referred to as baseband, carrier-free or short pulse) receivers was patented in 1972 by Ken Robbins while at the Sperry Research Center, U.S. Pat. No. 3,662,316. This receiver utilized a "dispersionless" broadband transmission line antenna together with a biased tunnel diode located in the transmission line for detecting the total energy in a pulse and expanding the resultant output in the time domain so that conventional, lower speed circuitry may be used for processing. The tunnel diode was biased to operate as a monostable multivibrator as disclosed in 1962 in Gentile, S. P., Basic Theory and Application of Tunnel Diodes, Van Nostrand, N.J., ch. 8 "Pulse and Switching Circuits" (1962). The receiver took advantage of the tunnel diode's unique characteristic of changing state when the area under the current vs. time envelope, i.e., the charge carriers passing through the device, exceeded a prescribed number of picocoulombs. This change in state yielded a recognizable, detectable event or output voltage. Sperry's tunnel diode detector (TDD) receiver was used in a number of applications including baseband communications, liquid level sensing, object detection and radar. It was soon observed, however, that the Robbins TDD was subject to operating point bias drift due to temperature and power supply fluctuations. This bias drift impacted negatively the system's overall sensitivity and increased the false alarm rate.
In 1976, Nicolson and Mara introduced a constant false alarm rate (CFAR) circuit to the tunnel diode detector receiver that is described in U.S. Pat. No. 3,983,422. The CFAR circuit employed a logic circuit that sampled noise dwells and data dwells to dynamically adjust a variable threshold of the tunnel diode. This feedback circuit operated in such a manner that the false alarm rate, as measured by the number of hits received due solely to noise during a fixed time interval, was held constant regardless of temperature fluctuations, power supply voltage changes, device aging, etc. The CFAR receiver was utilized in the development of baseband speed sensing, collision avoidance, and radar docking prototypes.
In 1987, an anti-jam circuit was introduced into the CFAR receiver. This is described in U.S. Pat. No. 4,688,041. Since the baseband receiver was extremely broadband, with typical bandwidths of hundreds of MHz to GHz, it was found to be extremely susceptible to in-band interference and jamming since the tunnel diode circuit could not distinguish between valid and unwanted signals. Such in-band signals caused a significant reduction in receiver sensitivity by causing the CFAR loop to back-off the sensitivity of the tunnel diode detector. The anti-jam circuit disclosed in U.S. Pat. No. 4,688,041 used the jamming signal itself (if sufficiently strong), or else an internally switched continuous wave (CW) signal, as a local oscillator signal to heterodyne the incoming signal prior to detection. However, this anti-jam circuit proved to be ineffective in the presence of barrage (broadband) noise jamming or interference, and/or multiple in-band CW interfences. In the case of barrage noise, no reference frequency is provided by the interference with which to down convert the incoming signal, and the system reverts to single-conversion superheterodyne operation with an internal first local oscillator. The broadband noise is also down converted with the signal, and no anti-jam improvement is obtained. In the latter case of multiple in-band CW interferers, the circuitry will use one of these tones, or a linear combination depending upon the third order intercept properties of the design. In this case, the remaining tones are also heterodyned to near baseband and act once again as strong in-band jamming signals.
Also in 1987, U.S. Pat. No. 4,695,752 disclosed a narrow range gate added to the existing baseband CFAR receiver. The reduction in range gate size had the effect of reducing unwanted noise and interference by more closely matching the detector with the received pulse duration. The inventor of this patent purports to achieve nanosecond range gate intervals through the use of two Germanium (Ge) and a single Gallium Arsenide (GaAs) tunnel diode.
In 1994, U.S. Pat. No. 5,337,054 to Ross and Mara disclosed a coherent processing tunnel diode UWB receiver. These inventors claim to have improved tunnel diode detector receiver sensitivity by using a tunnel diode envelope generator to perform a superheterodyne conversion whereby the available charge for triggering the tunnel diode is maximized. Ross and Mara considered only single pulse ultra wideband detectors; i.e., detectors which make a binary, or hard, decision (Logic 1 or Logic 0) at every sampling instant. However, their patent discloses a sliding average of detector hits, noise dwell or data dwell, in any group of thirty-two consecutive periods (col. 4, lines 35-39). Averaging of all hits, including data dwells, provides an average of the noise dwells which is skewed because of the inclusion of the data dwells. Moreover, to reduce the effects of the skewing, a large number of noise dwells must be detected for each data dwell detected, ultimately reducing data rates.
There have been other patented UWB receiver designs in which a multiplicity of pulses (typically several thousand) are first coherently added, or integrated, before a binary (bit) decision is made (e.g., U.S. Pat. Nos. 5,523,760; 4,979,186; and 5,363,108). The UWB detectors of the present invention do not require coherent addition of a multiplicity of pulses, but rather have sufficient sensitivity to operate on a single pulse basis.
Only false alarm rate is typically computed by previous UWB receiver/processor designs, and thus the system bit error rate (BER), and accordingly the receiver operating characteristic (ROC) are unknown. In practice, the tunnel diode bias is "backed off" from the CFAR level to reduce the BER to an acceptable level. Unfortunately, since the BER is a very sensitive function of the tunnel diode bias level, this can result in a significant reduction in receiver sensitivity to achieve a desired BER.
As disclosed in U.S. Pat. No. 3,662,316, in a tunnel diode UWB receiver, the tunnel diode changes state whenever the accumulated charge on the device exceeds a given threshold. Mathematically, the performance of the tunnel diode detector in additive white Gaussian noise (AWGN) can be described by the following set of equations: ##EQU1## where P.sub.d is the probability of detection, P.sub.fa is the probability of false alarm, s(u) is the received UWB waveform, n.sub.w (u) is additive white Gaussian noise with double-sided power spectral density N.sub.0 B, B is the detection signal bandwidth, T is the diode dwell sensitivity interval, and T.sub.h is a threshold value.
While previous designs of the CFAR tunnel diode receiver have functioned reliably as an ultra wideband single pulse detector, their use in modern communication and radar applications have presented numerous drawbacks:
1. The prior art designs remain susceptible to in-band interference and jamming, particularly broadband or barrage noise jamming and multiple CW interferers. PA1 2. The requirement to continuously adjust bias to the tunnel detector to maintain a given constant false alarm rate (CFAR) conventionally requires a minimum number of noise dwells to take place for each data dwell--typically thirty-two or more noise dwells for each data dwell--to achieve false alarm rates less than a few percent. This severely restricts the maximum data rate at which a single detector can operate since data and noise dwells must operate at different time intervals. In addition, the speed at which the tunnel diode detector can respond to sudden changes in the electromagnetic environment is limited. Hence, impulsive noise (which is nearly always present) can create burst errors in the data stream, corrupting data integrity. PA1 3. Receiver sensitivity is conventionally backed-off to achieve a desired BER, providing an UWB receiver which has reduced distance capability and slower data rates.